Electrorefining processes have been used to recover high purity metal or metals from impure feed material and more particularly to recover uranium and plutonium from spent nuclear fuel in a molten salt electrolyte. In the electrorefining process spent nuclear fuel forms the anode. The uranium in the spent fuel is separated from fission products and collected at the cathode through the electrorefining process. Controlling the morphology of uranium metal, which is the major constituent of spent nuclear fuel, deposited at the cathode has been a challenge for the electrorefining process.
FIG. 1 shows a sectional view of an engineering scale Mark-V (Mk-V) electrorefiner 10 operated at the Materials and Fuels Complex (MFC) site of the Idaho National Laboratory (INL) to process spent blanket fuel from the Experimental Breeder Reactor II reactor. The design and operation of the Mk-V electrorefiner is described in “Uranium Transport in a High-Throughput Electrorefiner for EBR-II Blanket Fuel”, Rajesh K. Ahluwalia, Thahn Q. Hua, and DeeEarl Vaden, Nuclear Technology, Vol. 145, pp 67-81, January 2004. The Mk-V electrorefiner comprises a metallic vessel 12 preferably constructed of an iron alloy. Within the vessel 12 is an electrolytic salt 14 such as LiCL-KCl eutectic with up to 6 wt % of UCl3. Vessel heaters 15 are used to maintain the electrolytic salt 14 at an operating temperature of approximately 500° C. Multiple anode/cathode modules (ACM) 16 are submerged in the electrolytic salt 14. A stirrer assembly 20 is disposed within the vessel 12 to maintain a flow of the electrolytic salt 14. Rotating contractors 22 provide for the rotation of the anode 35 within the anode/cathode modules 16.
Multiple concentric cathode tubes 26 within an ACM 16 are shown in FIG. 2 and FIG. 3. Also shown in FIG. 2 and FIG. 3 are multiple scrapers 32 positioned on the multiple anode baskets. The scrapers 32 are used to remove the built up uranium deposit on the cathode tubes 26 when the anodes 35 are rotating in the direction of the arrow shown in FIG. 3. As shown in FIG. 2, product collection bucket 34 is disposed at the bottom of the ACM 16 to collect the uranium deposit that is scraped off of the cathode tubes 26.
During the operation of the Mk-V electrorefiner, uranium in spent fuel is electrochemically dissolved and collected over many cycles, depending on the amount of fuel loaded in the anode baskets. Each cycle consists of three steps: (1) a direct-transport (DT) step in which uranium dissolves from the rotating anode basket and deposits on the cathode tube; (2) a cathode stripping step in which the polarity is reversed to electrotransport material on the cathode tube back to the anode basket; and (3) a wash step to physically dislodge material that may be been held up between the anode basket and cathode tube. Simulated cyclic variation of current and voltage during operation of the Mk-V electrorefiner is shown in FIG. 12 of the referenced Ahluwalia et al. publication.
A disadvantage of the Mk-V electrorefiner concentric anode-cathode design is that the uranium deposit does not continuously fall off the cathode as desired. Electrical shorting caused by the jamming of uranium deposition between the anode and cathode tubes has been frequently observed. The stripping and wash steps described above, and the use of scrapers to remove the deposited uranium from the cathode for collection in the product collection bucket, limit the efficiency and throughput of the electrorefining process.